1. Field of the Invention
This invention relates to apparatus for converting mechanical energy to electrical energy on an electrically powered vehicle and more particularly, to apparatus for mechanically generating alternating current power and rectifying the generated power to variable voltage direct current power for propelling the vehicle at a preselected speed.
2. Description of the Prior Art
Conventional direct current powered vehicles, such as mine haulage vehicles, commonly known as "shuttle cars" are supplied with continuous power through a trailing cable that connects the remote direct power source with the machine load. Power from the trailing cable is directed to the traction motors which are driven to propel the vehicle. The motors run at a single speed and, therefore, do not permit speed adjustments unless the vehicle drive train includes a multi-speed transmission. Subsequently, the operator must control the speed of the vehicle by depressing a foot pedal to turn on the motor. When the vehicle has reached the desired speed or the maximum permissible speed, the operator removes his foot from the pedal. The motor is thus turned off and the power to the motor is terminated. The vehicle is permitted to "coast" to a lower speed. By depressing the foot pedal, the motor is again energized and the speed increased.
This method of maintaining the speed of the vehicle within a desired range by "jabbing" the foot pedal subjects the motor to a high starting current which produces undesirable heating of the motor and trailing cable. Furthermore, the high starting current produces excessive motor torque that subjects the gear train and drive shafts to severe mechanical shock and high accelerational loads. The ultimate outcome of controlling speed of the vehicle by jabbing is poor operating performance and high vehicle maintenance.
A starting resistor is commonly utilized to connect the series field motor with the power source to reduce the high inrush or current on starts. The contacts of a time delay relay are usually connected across the starting resistor to cut in or out a portion of the starting resistance or the entire resistance from the motor. In this fashion, the presence of the resistance in the motor circuit reduces the acceleration of the motor. Then, after a predetermined time, the contacts are closed and the current bypasses the resistance and the motor accelerates to full speed.
To reduce mechanical shock to the gearing and shafts and provide soft starting and stopping capability, a chopper circuit has been utilized for D.C. battery powered shuttle cars. The chopper circuit includes a motor and a silicon controlled rectifier that are serially connected to the direct current voltage source. The rectifier and the motor are also included in a resonant circuit that comprises additional rectifiers and an inductor and capacitor. A transient signal is established by turning the rectifiers on and off to thus allow the capacitor to charge and discharge and force the current through the motor to zero. The resultant effect is a pulsating current with modulation.
To avoid the problem of turning the traction motors on and off to control the tramming speed of A.C. and D.C. mine shuttle cars, one approach has been to supply the traction motors with one of three available voltages. A multiconductor trailing cable connects the machine load to a remote power source to supply the vehicle with three phase, 60-cycle alternating current power in the range of 480 volts. One or more traction motors are connected to the high alternating current voltage through a diode rectifier bridge and a transformer. The transformer primary winding is connected to the multi-conductors and energizes the secondary transformer winding. Taps are provided on the secondary winding and a normally open contactor connects each tap to the diode rectifier bridge. Thus, by selectively closing the contactors, three voltage levels are available for controlling the speed of the traction motors. A problem is encountered, however, by the low frequency of the voltage and the poor motor performance resulting therefrom.
There is need for apparatus that supplies high frequency alternating current power to the power equipment of an electrically operable vehicle for operating the equipment at a controlled speed and does not require the use of a trailing cable to supply the machine with power from a remote source. While it has been suggested by the prior art systems to control the speed of the power equipment, the prior art systems require the utilization of the trailing cable and either alternatingly starting and stopping the motors or switching contactors of transformer taps to adjust the motor speed.